1. Field of the Invention
The present invention relates to a scanning optical device used in a laser beam printer, a laser facsimile apparatus, and the like.
2. Related Background Art
FIG. 1 is a plan view of a conventional light deflection scanning device used in an image forming apparatus such as a laser beam printer, a laser facsimile apparatus, and the like. A semiconductor laser light source unit 2 is fixed on the side surface of an optical box 1, and a cylindrical lens 3 and a rotary polygonal mirror 4 are arranged in front of the semiconductor laser light source unit 2. Imaging lenses 5 and 6, and a reflection mirror 7 are arranged in turn in the reflection direction of the rotary polygonal mirror 4, and a rotary drum (not shown) is arranged via an aperture portion 1a formed on the optical box 1 in the reflection direction of the reflection mirror 7. A mirror 8 is set at the position where some light components of light deflected and scanned by the rotary polygonal mirror 4 are incident, and a focusing lens (to be referred to as a BD focusing lens hereinafter) 9 for focusing a light beam to obtain a scanning start signal, and a photodetector (to be referred to as a BD detector hereinafter) 10 for obtaining the scanning start signal are disposed in the reflection direction of the mirror 8.
In this device, the cylindrical lens 3 is arranged so that its center nearly matches the center of the optical axis of the semiconductor laser light source unit 2. The bottom surface of the cylindrical lens 3 contacts a reference surface 1b formed on the optical box 1, and an end face 3a of the cylindrical lens 3 contacts a butting reference surface 1c formed on the optical box 1 to extend in a direction parallel to the optical axis direction of the semiconductor laser light source unit 2.
Upon assembling the individual members to the device, the spot size in the sub-scanning direction on the surface of the rotary drum may often deviate from the design value due to factors such as dimensional errors on the optical layout, shape errors of the lens itself, and the like, and for this reason, focus adjustment must be done by moving the cylindrical lens 3. Hence, while the end face 3a of the cylindrical lens 3 is pressed against the butting reference surface 1a on the optical box 1, the lens 3 is slidably moved on the reference surface 1b on the optical box 1. During this slidable movement, the spot size on the surface of the rotary drum is observed, and the cylindrical lens 3 is fixed on the optical box 1 by, e.g., a photosetting adhesive or the like at the position of the minimum spot size.
With this arrangement, a light beam emitted by the semiconductor laser light source unit 2 is irradiated onto the rotary polygonal mirror 4 while being linearly focused by the cylindrical lens 3. Most light components of the light beam deflected and scanned by rotation of the rotary polygonal mirror 4 are irradiated onto the reflection mirror 7 via the imaging lenses 5 and 6, and are reflected by the reflection mirror 7. The reflected light beam reaches a photosensitive member on the rotary drum (not shown) via the aperture portion 1a of the optical box 1. The light beam that has reached the photosensitive member forms an electrostatic latent image on the photosensitive member since it is main-scanned upon rotation of the rotary polygonal mirror 4 and is sub-scanned upon rotation of the rotary drum. Some light components of the light beam deflected and scanned by rotation of the rotary polygonal mirror 4 enter the BD detector 10 via the mirror 8 and BD focusing lens 9.
However, in the above-mentioned prior art, since the cylindrical lens 3 and BD focusing lens 9 are located at independent positions, problems of a size reduction of the device, improvement of productivity, a cost reduction, and the like are posed. To solve these problems, an arrangement that integrally forms the cylindrical lens 3 and BD focusing lens 9 has been proposed. However, in this arrangement, if the optical axes of the cylindrical lens 3 and BD focusing lens 9, which are integrally formed, are set to be parallel to each other, since the laser beam once emitted directly returns to the semiconductor laser light source unit 1, an image write start position detection signal (to be referred to as a BD signal hereinafter) cannot be detected. Furthermore, when focus adjustment is done by moving the cylindrical lens 3 in the optical axis direction, the optical axes between the BD focusing lens 9 and BD detector 10 deviate from each other, thus shifting the reception timing of the BD signal.